Comparison of traditional field retting and Phlebia radiata Cel 26 retting of hemp fibres for fibre-reinforced composites

Abstract

Classical field retting and controlled fungal retting of hemp using Phlebia radiata Cel 26 (a mutant with low cellulose degrading ability) were compared with pure pectinase treatment with regard to mechanical properties of the produced fibre/epoxy composites. For field retting a classification of the microbial evolution (by gene sequencing) and enzyme profiles were conducted. By phylogenetic frequency mapping, different types of fungi, many belonging to the Ascomycota phylum were found on the fibres during the first 2 weeks of field retting, and thereafter, different types of bacteria, notably Proteobacteria, also proliferated on the field retted fibres. Extracts from field retted fibres exhibited high glucanase activities, while extracts from P. radiata Cel 26 retted fibres showed high polygalacturonase and laccase activities. As a result, fungal retting gave a significantly higher glucan content in the fibres than field retting (77 vs. 67%) and caused a higher removal of pectin as indicated by lower galacturonan content of fibres (1.6%) after fibres were retted for 20 days with P. radiata Cel 26 compared to a galacturonan content of 3.6% for field retted fibres. Effective fibre stiffness increased slightly after retting with P. radiata Cel 26 from 65 to 67 GPa, while it decreased after field retting to 52 GPa. Effective fibre strength could not be determined similarly due to variations in fibre fracture strain and fibre-matrix adhesion. A maximum composite strength with 50 vol% fibres of 307 MPa was obtained using P. radiata Cel 26 compared to 248 MPa with field retting.

The Microbiology of Hemp Retting in a Controlled Environment: Steering the Hemp Microbiome towards More Consistent Fiber Production

“…While the results do not support large shifts in the population according to time or treatment, a notable increase in the phylum Bacteroidetes, especially the genus Chryseobacterium, was observed in the final time points for HM and SHM treatments, but not LM, indicating moisture levels may influence this population over the course of retting. These results are in agreement with field studies of hemp and flax dew-retting, where similar microbial profiles were reported, and Bacteroidetes was found to increase toward the end of the retting process [15][16][17]. Previous analysis of the bacterial population changes that occur within retting hemp stalks showed an increase in the ratio of Bacteroidetes from mid-retting to full-retting conditions [18].…”

“…Specifically for fiber use, problems hinge around issues of scale and economy, in which the retting and decortication process has been described as a "bottleneck" for supplying consistent product to industry manufacturers [2,4,20,21]. Research addressing this problem has focused broadly on two areas: alternatives to field-retting such as solid-state fermentation with specific organisms and post-harvest treatments that focus on improving the end product without regard to the logistical and economic considerations, or adjusting agronomic practices such as timing of planting and harvest, turning in the field, and variety selection [6,15,17,[22][23][24]. Fungal species are predicted to play a role in retting, by both their own degradation abilities and the process by which they facilitate entry of surface microorganisms past the cuticle, and have been a target for attempts to manipulate retting outcomes [17,25].…”

“…Research addressing this problem has focused broadly on two areas: alternatives to field-retting such as solid-state fermentation with specific organisms and post-harvest treatments that focus on improving the end product without regard to the logistical and economic considerations, or adjusting agronomic practices such as timing of planting and harvest, turning in the field, and variety selection [6,15,17,[22][23][24]. Fungal species are predicted to play a role in retting, by both their own degradation abilities and the process by which they facilitate entry of surface microorganisms past the cuticle, and have been a target for attempts to manipulate retting outcomes [17,25]. These efforts have all contributed useful information, but moving towards cost-effective retting practices that result in consistent fiber production with the specific qualities required by industry will require substantially more research from a variety of directions, such as traditional plant breeding for desirable field retting qualities (high cellulose, low lignin and pectin), molecular research into genes that affect fiber yield and quality, and microbiological investigations into specifically how the microbial profile associated with bast fiber crops is recruited and maintained throughout the plant's life as well as the retting process [20,[26][27][28][29].…”

A Comparative Study of the Effect of Field Retting Time on the Properties of Hemp Fibres Harvested at Different Growth Stages

“…This colour transition is typical during field retting, even for other natural fibres such as flax fibres [11,28]. The colour variation from yellow to grey for the fibres is related to the development of microbial communities (fungal and bacteria [29][30][31] at stem surface. In addition, during field retting, black spots appeared and increased on the stems surface until the colour of the stems became black.…”

“…Therefore, when fibres are extracted from the retted stems, micro-residuals xylem (shives) could be still bounded to the fibres and overestimate the real variation of lignin during retting treatment. This variation in biochemical composition is due to both biofilm growth (ESEM investigations) and the metabolic activity of microorganisms during hemp fibres retting [29,30].…”